The Newark Basin

The geologic feature called the Newark Basin is foundation for the most heavily
urbanized and industrialized portion of New Jersey (Figure 87). The basin extends
south and westward from the lower Hudson River region, across northern and central
New Jersey, and continues southward into Pennsylvania where it diminishes and
then enlarges southward into the Gettysburg Basin. The half-graben structure
of the basin is reflected by the gentle westward dip of the strata at about
15 (Figure 88). Possibly as much as 11,000 feet of sedimentary rock and volcanic
material of Mesozoic age exists beneath the surface on the western side of the
basin. A great border fault system defines the western side of the basin. One
of these, the Ramapo Fault, extends southward from the Hudson Highlands region
east of the Hudson River (in the vicinity of the Indian Point Nuclear Power
Plant). In New Jersey, Interstate 287 roughly follows the trace of the fault
southward to near its intersection with Interstate 78 where other faults define
the western border of the basin. This fault is responsible for the escarpment
of the Ramapo Mountains of the Highlands that rise on the western side of the
fault.

Figure 87. Map with field localities in the Newark Basin, New York, New
Jersey, and Pennsylvania (after Drake et al, 1996 and Lyttle & Epstein
1987).

Figure 88. East-to-west cross-section of the Newark Basin along I-80.

Figure 88 illustrates the general structure and stratigraphy of the Newark
Basin along Interstate 80. The three lower formations, the Stockton, Lockatong,
and Passaic Formations are Late Triassic red beds. Volcanic intrusions of Jurassic
age, including the Palisades Sill, has intruded within these formations. The
1st, 2nd, and 3rd Watchung Mountains, east to west respectively, have been renamed
Orange Mountain, Preakness Mountain, and Hook Mountain, respectively, and provide
the names for the Jurassic age surface flows responsible for their origin. (The
word, Watchung, comes from the Lenape Indians meaning high hills.) The sedimentary
formations between and above these flows, the Feltville, Towaco, and Boonton
Formations, represent basinal sediments deposited in the Early Jurrasic. The
gentle westward dip of the strata probably continued to develop after the youngest
formations were deposited.

The traprocks within the Newark Basin display subtle differences in both composition
and physical appearances from one location to the next. The composition of the
rock varies from ultramafic (rich in olivine) to dioritic, depending on the
amount of dilution of basaltic magma with quart-rich material derived from the
continental crustal rocks it invades and partially melts as it migrated toward
the surface. Where the magma cooled more slowly beneath the surface within volcanic
stock and sills, the more moderately crystalline textured rock is called diabase.
The dominant igneous rock-type is the fine-grained extrusive form of basalt
that formed as volcanic material extruded on the surface and cooled as flows.
However, the physical character of the basalt varies considerably form one area
to the next, depending on how the rock cooled.

Where massive amounts of material erupted on a flat surface, the lava flows
generated were locally as much as 200 feet thick. As the lava cooled, it shrank
and cracked. In places where homogeneous lava cooled at an uniform rate vertical
polygonal columns formed. If the lava flowed down a sloping surface, the flows
tend to be thinner, and they cooled more rapidly and unevenly. On modern volcanos,
blocky-textured flows (called "aa" after Hawaiian usage) have an uneven
texture which forms as material that has cooled on the surface is mixed downward
into the migrating flow. This causes inconsistent cooling patterns within the
flow, resulting in an irregular fracture pattern within the resulting lava rock.
Flows where the lava is much hotter and more fluid tend to spread out and cool
with a smooth, billowy or ropy surface (called "pahoehoe"). This hotter
lava tends to spread out as fingers of lava that cool as irregular pods called
"pillows." Pillows that form underwater tend to be more rounded, more
fractured, and have thicker glass-textured rinds than pillows that form on land.
All types of flows can be recognized in the traprocks of the Newark Basin.

The character of the geologic structure of the Newark Basin is quite apparent
when driving westbound on any of the interstate highways that cross the basin.
Beautiful exposures of fanglomerates, brownstone, and traprock can be seen in
road cuts along the highways, but unfortunately, they are not safe or legal
to stop and examine. The sites described below include a mix of scenic natural
areas on public lands and popular geologic field trip destination sites in the
Newark Basin region.